June, 1957
I-TIGH P R E S S U R E 'I)TRSOCTATION O F T H E
URANIUM-HYDROGEN SYSTEM
703
tetrasiloxane, the Minnesota Mining and Manufac- for helpful comments, and the Atomic Energy turing Company for the f-heptane, Dr. R. L. Scot,t Commission for the support of the work.
HIGH PRESSURE DISSOCIATION STUDIES OF THE URANITJM HYDROGEN SYSTEM1 BY G. G. LIBOWITZ? AND THOMAS R. P. GIBB,JR. Contribution No. 844 from the Department of Chemistry, Tilfls University, Medford, Mass. Received Februarv 1, 1967
Pressure-composition isotherms were ohtained for the uranium-hydrogen system over the temperature range 450 t o 650" at pressnres up t o 65 atm. in a stainless steel apparatus designed for high tem erature hi h pressure measurements. The experimentdy determined plateau presRures are com ared with values extra okted from ?ow temperature stiidies of Spedding and Flot,ow and Abraham. A value of -30.3 !cal./mole was 0btainecf)for the heat of formation of UH1 from the dissociation pressure data.
Introduction The prcssuro-temperature-composition relations in the uranium-hydrogen system have been investigated a t temperatures below 430" by Speddinga and Flotow and Abraham.4 These investigators found stoichiometric uranium hydride to have the formula UHS. At ttemperatures above 430" the dissociation pressure of the hydride becomes greater than one atmosphere, and the usual glass-quartz apparatus can no longer be used to study this system. Mogard and Cabane6 have measured the dissociation pressures of uranium hydride a t high pressures, and Gibb, MciSharry and Kruschwitze have made R high pressure study of the high hydrogen composition region of this system. It was the purpose of this investigation to study the uranium-hydrogen system a t high pressures and temperat,uresover the whole range of hydrogen compositions from uranium metal t o fully hydrided UHa in a stainless steel apparatus especially constructed for these investigations. Experimental Apparatus.-The
apparatus, shown schematically in Fig.
1, consisted of a sample container, resistance furnace with
temperature measurement and control systems, high pressure measuring systems, low pressure glass volume calibration system, and a hydrogen purification train. The tubing throughout, the high ressure part of the apparatus was Type 31G stainless steer, in. 0.d. and '/a in. i.d. All connections were made with "Koncentrik" fittings with steel seats having Teflon ring gaskets. Valves A, B and C were Hoke Needle Valves with V oint spindle. The other valves in the system are Hoke diapiragrn-type packless valves. The sample container was made from Type 316 stainless Rteel. The bottom was machined from flat stock and the main portion was bored and machined from Nolid rod. The parts were put t)ogether by at,ornic hydrogen welding. The cavity in the container was 7 in. 11y 1.5 in. diameter. The wall thickness was i n . A lead-in tube of standard weight ( I ) This rcsearch was supported by the Atomic Energy Comrnisaion. Presented a t the 130th Meeting of the American Chemical Society, At,lantic City, New Jeraey, Sopt,ernber 21, 19513. (2) Materiala Rcsearch Group, Atomice Int,ernatiooal, Canoga Park, California. (3) F. H. Rpedding, A. S. Newton, J. C. Warf, 0. Johnson, R . W. Nottorf, I . B . Johns and A . H. Daane, Nucleonics, 4, 4 (1940). (4) H. E . Flotow and B . M. Abraham, Report AECD-3074, declassified Jan. 30, 1981. ( 5 ) H . Mogard and G . Cabane, Rai. n e t . , 61, 617 (19.54). (6) T. R. P. Gibb, Jr,, J. J. McSharry and H . W. Kruechwitz, J . Am. Chum. &e., 74, 6203 (1952).
3/* in. Rt,ainle,ssstrel tuhing with a Koncrntrik fitting at tho top, perinit,t,rtl t,lie container t,o he attached to the high pi'eaRure liue. Hy dosing valve D , t8hebomh could be rcmoverl from the furnace withoiit coiit8aminatingthe s:imple. The resiatance hentw consisted of two sections: the main winding and an auxilia1.y hoat~rrsitriated near the top of tho furnace t,o eliminate thermal gradients. The main part of the furiiace was a Norton Alundum core, 10 in. long and 4.5 in. in diameter wound with 16 gage Kanthal wire. The snrnrile container rested on a steel nedestal mounted at the bottom of the furnace. The core waR surrounded by firebrick at, the sides and bottom, which in turn was surrounded by a galvanized iron can wrapped with asbestos. The t,op of the furnace was covered with a slab of in. transite containing openings for the sample container and thermocouples. The srparately controlled auxiliary heater consisted of coiled Kanthal wire supported by three porrelain rods suspcndcd from the bottom of the trnnsite slab. In this way, the t,op of the bomh received additional heating to rompensate for the heat loss due t o thermal conductivity. Tempcratures were measured with two calihrakd ChromelAlun~elthermocouplcs, and controlled to f0.5"by means of R photoelectric conttrol circuit. The two thermocouples were strapped to the sample container with aslmtos tape, and were so situated that they measured the temperatures at the top and at the bottom of the sample when it, was fully hydrided. I3y inserting a thermocouple inside the container under conditions similar to those used in mn,king a run, it wns found that the inside and outside temperatures differed by one to two degrees over the range studied. This difference was taken int,o account in setting the temperature controller. The pressure of hydrogcn over the sample was measured with a Brown Instrument Co. Recording Pressure Gauge. This was a multJipleBourdon spiral gage with three separate recording u n h of 0-20, 0-100 and 0-1000 p.s.i. The 0-20 and 0-100 p.s.i. units were connected to the manifold through valves which could he closed when the pressure became higher than these maximum values. During use, it was found that these pens were accurate to =t0.5% of their full scale reading. The low pressure Pyrex glass system wns separated from the high premure portion of the apparatus by a Hoke needle valve, C . The glms system included two bulbs whose volumes were 5.535 and 12.97 liters. These were used for measuring the volumes of hvdrogen gas removed from the samplc during dehydriding 'runs. The glass system also contained a smnll calibrated bulb for calibrating volumes in the system and an open end mercury manometer. Materials.-The ur:tnirim metnl used WXR high purity Reactor grade (>99.9% U ) metal furnished by the Atomic Energy Commission. A 230-gram hlock of metal was cleaned in I : 1 HNOI, 0.1 N HCI, water, isopropyl alcohol and anhydrous ethyl ether in turn and transferred to the sample container under an argon atmosphere. Hydriding was carried out at about 200" and under a hydrogen pressure of 500 p.s.i. In this way, uranium hydride, having the stoichiometric formula UH3 at room temperature, could be prepared. The hydrogen was purified by passing over
Fig. 1.-High
prcssure apparatus for dissociation studies.
drierite, hot uranium metal getter, and finally through a glass wool trap to remove any getter particles. Experimental Procedure.-In a typical deh driding run, the uranium hydride was heatcd to the desireitemperature under a pressure of hydro en. After the system reached equilibrium ( i . e . , pressure $id not change more than 0.5% of the full scale reading of the en in the period of one hour), the pressure was recorded. +he composition of the hydrogen in the sample was then decreased by bleeding hydrogen into the calibrated bulbs, F and G , and the pressure again recorded after it had become constant. By successive measurements in this way, pressure-com osition isotherms a t 60" intervals from 450 to 650" were oitained. The compositions were calculated by subtracting the S.T.P. amount of hydrogen above the sample and the amount of hydrogen bled into the glass system from the amount of hydrogen originally present in the container and the amount of hydrogen in the fully hydrided UH3, the amount of hydrogen in each case being calculated from appropriate volumes, pressures, and temperatures. For these cttlculations, the non-ideality of the gas law a t high pressures, as well as the hot and cold volumes of the sample container were taken into account. At least two dehydriding runs were made a t each temperature. For hydriding runs, the sample was completcly dehydrided to uranium metal by evacuating a t 500". With the sample under vacuum, valve D wns closcd and the line hetween valves D and H was filled with hydrogen from the hydrogen storage c linder a t a pressure which could be read on the recorder. &he sample was brought to the desired temperature, and valve D was opened to permit hydrogen to flow into the sample container. After the svstem reached equilibrium, the pressure was recorded, valve b was closed again, and another volume of hydrogen was added by the same procedure. From the change in pressure on opening valve D, the amount of hydrogen added and the comosition was calculated. By repeating this procedure, the pow composition and plateau portions of the isotherms could be obtained. At high pressures, the difference between dissociation pressure and the pressure in the manifold from D to H was too low to permit large enough increments of hydrogen to be added., so that hydriding runs were discontinued before the region,of rapidly ascending pressures a t high hydrogen compositions. A t temperatures above 550", it was found that the diffusion of hydrogen through the steel was no longcr negligible. The diffusion rate was measured a t each temperature as a function of the pressure, and these data were used to correct the compositions for diffusion 1 0 s ~ . Although the temperature was controlled to within a few tenths of a degree centigrade with a photocell circuit, the uncertainty in temperature may be as high as 1 2 " because of slight thermal gradients in the sample. As statcd above, the accuracy of a pressure measuring unit was f0.5% of its full scale reading. Under the worst conditions, therefore (reading 100 p.8.i. on the 0-1000 p.s.i. unit), theerror could be as high as f 5 % , although it was usually much less. The error in plateau pressure due to uncertainty in temperature varies from 4 ~ 4 for 7 ~the 450" isotherm to &2%for the G50" isotherm. In gcncral, therefore, it can be said that the error i n presmre is no higlicr than f G % and uaually less. Be-
muse the composition rrror is compounded with each removal of hydrogen, the composition a t low hydrogen contents i R I r s s arcuratc than a t high hydrogen contcrit8. Tn the region of rapidly rising pressures a t the high cornposition end of the isotherms, the precision in composition i R d~0.01 H/U, so that the up cr limit of the platcau can be determined to 1 0 . 0 1 H/U. Rowever, the shapes of the curves in this region can be determined more precisely since the relntive error of one experimeiital point to the next varies from f0.009 H/U at high pressures to 40.001 at the plnteau pressure. Notwithstanding, the lower limit of the plateau cannot be located to better than zkO.05 H/U. By shifting the low hydrogen composition portion of the isotherms RO that the steeply :loping part intercepts the point of zero h drogen composition a t zero pressure, the lower limit of the fatea. can be determined somewhat more recisely. This as bcen done for the isotherms shown. &r the two hydriding runs, the error in hydrogen composition was somewhat larger. I n the 500' isotherm, the error in composition a t the low hydrogen composition end of the isotherms was only 10.001 H/U. As the hydrogen composition increased in the plateau region, the error increased to k0.03 II/U near the upper limit of the plateau. The compositions in the hydriding run a t 650" were much less precise, the errors being *0.01 H/U a t the low hydrogen composition and 1 0 . 1 H/U in the plateau region. This large error was due to the fact that the dissociation pressure was so high a t this temperature, that the difference between hydriding pressure and dissociation pressure was small, thereb necessitatin a large number of hydrogcn additions in or&r to increase t i e hydrogen content of the sample appreciably.
Results and Discussion The pressure-composition isotherms obtained are shown in Figs. 2 and 3. The open circles represent dehydriding runs and the black circles hydriding runs. Within experimental error, there seems to be no hysteresis between hydriding and dehydriding in this system at temperatures above 450". Hysteresis was obtained by Speddinga a t lower temperatures. As hydrogen is removed from the fully hydrided uranium hydride, the pressure falls rapidly until a constant pressure plateau is reached due to the dissociation of uranium hydride and the formation of two solid phases; puranium hydride and a-uranium metal. When almost all the hydrogen has been removed, the hydride phase disappears and the pressure again decreases sharply, indicating a single solid phase consisting of the metal containing dissolved hydrogen. The plateau pressures obtained in this investigation are compared with the plateau pressures calculated by extrapolating the low temperature (below 430") work of Spedding3 and Flotow and Abraham4 in Table I. The errors shown are the estimated maximum errors. The plateau pressures agree within experimental error with those obtained by the extrapolation of Flotow and Abraham's equation a t 450 and 500". However, at higher temperatures, it appears that the low temperature equation can no longer be used. The values obtained from the equation given by Mogard and Cabane5 for high pressures are also shown in this table. Their values are much lower than the present values, however, (except a t G50") and are even lower than the Flotow and Abraham values a t 450 and 500". Since their work was essentially a metallographic study of UH3, no details are given on the measurement of the pressures so that there is no indication of the accuracy of the work. If we consider the reaction U
+ 3/2112 --+
UH3
(1)
TARLE I nISSOClATlnN
I’RnRSIIRES
(A1.M.) OF
“C.
Spcddinr:
Ilotow and Abralialn
150 500
1.51 3.80 8.5 17.8 33.8
1.50 3.M 7.8 14.9 26.0
Trlllp.,
550
600 650
~ J R A N I I I L I ITYnRlDE :
hIoanrd and Calmno
1 ,733
3,35 7.5 15.5 20.2
This
rcscarch
1.44 f 0.06 3 . 5 7 f .12 8 . 4 f .4 16.3 f . 5 29.8 f . 9
t’hc hen,t of format#ion,A H , of t8hcuranium hydride can he nnlculat,ed from tjhr in t.cgrnt,rd form of t8he Van’t,TIoff cqu a t,’ion In P =
2AH 3RT
+ coiist,nnt
nmoatm
whcre P is the plateau pressure, T the almolute temperature and R is t,he gas constant. The equation for the log P I N . 1/7’ plot shown in Fig. 4 was dctmmined to lie log
P3tm =
-0:__ 4
o o u c o n ~ ~ o ~n- m ~m o YN,
Fig. 2.-Uranium hydrogen pressure-composition isotherms: open circles represent dehydriding data; closed circles repreaent hydriding data.
+ G.2G
*
from which a heat of formahion of -30.3 0.1 kcal./mole was o l h i n e d , as compared to -30.8 kcal./mole from Spedding’s datJa and -31.8 kcal./mole from the work of Flotow and Abraham. The most recent calorimetric determination by Abraham and Flotow’ is reported aa -30.35 f 0.03 kce,l./mole at room tcmperature. The excellent agreement, between the high temperature and low temperature value indicates that AH remains reasonably const,ant,with temperature. Some recent calculations8 indicatJe that the nonstoichiometry of uranium hydride is due to hydrogen vacancies in t,he lattice. The high composition ends of the isothermi.; shown in Fig. 3 can t,hen be int.erpret,ed in the following way. As hydrogen is wit,hdrawn from the syst,cm, vacancies are created in the lattice, and the pressure drops sharply until the saturation conceiitra,tion of vacancies is reached. At this point,, the lattice cannot, accommodate any more vacancies, and further removal of hydrogen causes the hydride to break down, or dissociate, thus forming a two-phase region consisting of uranium metal and uranium hydride containing the maximum number of hydrogen vacancies. As tjhe temperature increases, tho number of hydrogen vacancies which the lattice can accommodate inc.reases. If this is the case, the heat of formation calculated above is not for reaetion 1, hut rather for the reaction (1
+ r ) U + 23 H Z+Uj+zHa
where x varies with ttemperature and is equal to v / ( l - v ) , v being the fraction of vacant hydrogen sites in the Iatticc. However, since A H is constant witJh ternperatsure over the range studied as sho~7n isa 133 in Fig. 4, it is obvious that the presence of vacancies does not appreciably affect the heat of formation, Fig. 4.-Van’t Hoff isochore for uranium-hydrogen Ryst,em. and the value given for AH is also valid for z = 0, TABLE I1 or stoichiometric UH3. The maximum value of THE SATURATION CONCENTRATION OF HYDROGEN VACANCIES v at each temperat,ure is shown in Table IT. WI
i.18
110
ICI
I Y ~
1000/T*1.
(7) B. M . Ahraliain a n d 11. E. Iilotow, J . A m . Chem. S o r . , 7 7 , 1446 (1955). . . ( 8 ) G . 0.Lihowitz. srihniit,tivl for publicntion.
Temp., “C. V
I N URANIUM HYDRIDE 450 500 550 600 0.008 0.018 0.027 0.041
650 0.054
